MADISON (WKOW) -- The U.S. Supreme Court ruled Wednesday that police cannot search a cell phone without a warrant -- even if the phone belongs to someone who's been arrested.

In a unanimous decision, the justices ruled that modern day cell phones contain vast amounts of personal data and thus, while police are entitled to seize them from an arrested suspect, they must obtain a warrant before accessing a phone's contents.

The ruling does seem to allow for exceptions in the case of emergencies or imminent danger -- such as if a phone is being used to detonate an explosive device.

"This does seem to signal a fairly broad protection of security in terms of electronic information, in terms of electronic devices like this," said UW-Madison law professor Adam Stevenson. He said Wednesday's decision could have implications on whether police are able to search laptops or tablets as well.

"It's a little bit like if an officer were to search you and find a key - a physical key - and that key opened some room that contains your life story," Stevenson said. "(Police) couldn't use that key and open that door without a warrant."

Stevenson said it's likely Wednesday's ruling will only apply to future cases -- and will not be used retroactively to exonerate those convicted through unwarranted cell phone searches. But he said the ruling does open the door for people convicted in such cases to appeal.

Locally, the Madison and Middleton Police Departments said they both had previously made it policy to obtain warrants before searching a suspect's cell phone.

A spokesperson for the Dane County Sheriff's Department said it also obtains warrants before searching mobile phones.